This invention relates to curling irons, and more particularly to curling irons having a heating tube which has a flattened oval shape with depressions in selected surfaces.
Many devices are available for curling a person's hair by wrapping a tuft of hair strands around a heated cylindrical tube, and transferring heat from the tube to the hair. Commercially available curling irons often include such a tube secured to a handle, and a heating element inside the tube. A clamp extends along substantially the entire length of the tube.
The ends of the hair strands are grasped between the tube and the clamp and the hair strands are wrapped around the tube and over the clamp by twisting the iron. Heat which is transferred from the tube to the hair causes the hair to retain a spiral shaped curl when the clamp is released and the curling iron is removed. The shape and resiliency of the curls are determined to a significant degree by the temperature of the hair, the length of time that heat is applied to the hair, and the radius of the tube.
Optimum curling of hair strands occurs when the heating tube is at its selected operating temperature. When the tube is at its operating temperature, it stores heat energy which is used to curl the hair. The heating element restores heat energy to the tube as heat is absorbed by the hair and clamp or dissipates. However, as the curling iron heats hair strands to create curls, the absorption of heat by the hair strands cools the heating tube somewhat, especially since the tube is round and the hair strands absorb heat around the entire circumference of the tube. The drop in temperature is dependent in part on the amount of heat energy which the tube can store, and the amount of heat which the hair absorbs.
This cooling effect is undesirable because curls made when the heating tube is at a lower temperature take longer to make, and curls created at the lower temperature are not as sharply defined and resilient as curls made at the selected operating temperature, unless the length of time that heat is applied to the hair is increased. The thermal capacitance, or ability of the heating tube to maintain its operating temperature, can be increased by increasing the mass of the tube, but that can make the tube undesirably large or heavy. Positive temperature coefficient PTC thermistors, which can be used in heating the tube, provide improved heat recovery, but are relatively expensive and difficult to insulate and install in the tube, and may not reach a sufficiently high temperature for many applications. The operating temperature of the tube can be increased to store additional heat energy in the tube, but a higher temperature can be impractical and even dangerous under some circumstances. Thus, there is a need for curling irons which more effectively maintain the heating tube at or near its selected operating temperature.
Since the heating tubes just described are round, the entire hair strand is heated and curled, which requires substantial heat energy from the curling iron. Moreover, heat is not transmitted efficiently to the outside layers of hair surrounding the tube, because the hair strands are wrapped over the clamp, and much of the hair is wrapped upon itself. Thus, heat transfer to the outer layers of hair is reduced because the entire hair strands are heated, and the outer layers are not in direct contact with the heating iron tube. Heat transfer to those portions of the hair to be curled is also reduced because the clamp absorbs heat energy. Thus, there is a need for curling irons in which heat transfer to those portions of the hair to be curled is improved because the hair is not wrapped over the clamp or upon itself.
Curling irons with round tubes form substantially round, spiral-shaped curls. Such irons are not used to produce other types of new and interesting curls, including Z-shaped curls, as well as other types, which have a different appearance. Thus, there is a need for curling irons which form Z-shaped curls and curls having other new and interesting shapes.
Accordingly, one object of this invention is to provide new and improved curling irons for hair.
Another object is to provide new and improved curling irons which provide increased heat transfer to those portions of the hair to be curled by reducing the length of the clamp.
Still another object is to provide new and improved curling irons which better maintain the heating tube at a desirable temperature as curls are created by increasing the heat energy transferred to portions of the hair strands which are actually curled, and decreasing the transfer of heat energy to portions of the hair strands which are not curled.
Yet another object is to provide new and improved curling irons which utilize energy efficiently.